Photoshop CS5: If I only wish to select the black portion of the below image (to create a layer mask with it), how would I do so? It was generated by clicking "Make Selection" from a vector shape that's made up of several paths (each of the triangles, then the rectangle that encloses them). Thank you in advance!

As a related follow-up, how do I add to a selection? i.e. If I make one selection, how do I keep adding more parts to it? Necessary to use paths?

The answers are good and its a good question but for your particular problem why not just open it in illustrator and add a stroke and remove the fill. Since you already have it in vector that would be the best result
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Ryan♦Jan 30 '13 at 17:19

Hi Horatio. This is really useful, thank you, but two more q's based on it: 1. Is there a way to see which parts of a multiple-shape selection are "added" vs. "subtracted," i.e. Selected vs. removed from the selection? Also, 2. With my example above, this is a selection made from a workpath, so I'm not sure how the above would apply, as I am not making the selection from scratch... Is there a way to do it here?
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zakgottliebJan 31 '13 at 12:21

It is worth experimenting, but the ability to add and remove is tool based. You can easily switch tools without destroying the selection. If you need to break it into steps to get your head around, you can select a part you want to remove, create a temp layer, flood it with black and hide it. Deselect, make a new selection, and then ctrl+shift+click the icon of the temp layer to subtract the temp layer. etc. etc.
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horatioJan 31 '13 at 14:57

Once selected, consider the selection itself an object which can be worked on.
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horatioJan 31 '13 at 14:58

I'm sorry - I understand the suggestion, but does that mean this method is not possible if a selection already exists from a work path? To be clear, I already have the above selection from the vector shape - i do not wish to make another one - but I simply wish the parts in black to be selected, and nothing else... I'm not sure how to accomplish this based on your suggestion.
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zakgottliebJan 31 '13 at 17:26

When you choose a Selection tool (Marquee, Lasoo, Quick Selection tool)
you will find options for adding/substracting/intersecting with an existing selection in the Tool Options Bar(up).

Screenshot 1:
Screenshot 2:
In this screenshot I am first(#1) creating a simple sqare selection with the "New selection" option of the "Rectangular Marquee Tool".

After I have the existing selection I am substracting a part from it using the "Substract from selection" option of the "Rectangular Marquee Tool".

I hope this clears any confusion about my answer. Good luck!

UPDATE - 25/02/2013**:
What the asker, Zak833 is actually looking for is something like the Pathfinder Tool in Illustrator, but for selections not for Shapes... he is trying to use the Selection Tool in a matter that the Shape Tool usually works,

A kind of workaround:
First work with the shapes you are trying to alter then recreate the selection from the shape.
Since Photoshop CS6 there are some simmilar options when creating a new shape(see screenshot). So your workaround here would be something like this:
Again, first alter/modify your shape then remake your selection from that new modified Shape.

Thanks, Flavius. I understood his answer well, but the primary question is how to do this with sections of a pre-existing selection, such as the one above? I made this selection from a work path, but now I just wish to isolate the black sections. One answerer suggested Select -> Color Range, which I will probably do, but I was wanting to know if there was a way to do it with a similar method to the one outlined by horatio or yourself....
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zakgottliebFeb 1 '13 at 11:16

you use the "substract" option from the above mentioned and you take out what ever is extra in order to isolate the black sections... maybe I don't understand correctly what you are trying to achieve... I'll think about it
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Flavius FrantzFeb 1 '13 at 11:53

I guess I'm not explaining it well. That suggestion you just made would involve making a NEW selection in order to subtract... What if I wish to "select" part of the selection that has ALREADY been made and tell it to subtract from the selection behind it? i.e. If I did not want to alter the shape of the already-made selections, but still wanted one of them to subtract from another? Does it make more sense now?
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zakgottliebFeb 1 '13 at 16:40

OK. What you and horatio have suggested is great for controlling selection areas when you have not already got a selection in front of you to begin with. In the case that I've shown with the envelope, it is a selection that was made from a work path. I am looking to add/subtract parts of this already-existing group of selections from other parts in the group, rather than priming the selection tool to add or subtract in the way that yourself and horatio have suggested. The selections are already there, so how do I configure one of them to subtract from another, for example?
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zakgottliebFeb 4 '13 at 14:33